Making Sense of Biology
Nothing in Biology Makes Sense Except in the Light of Evolution, Theodosius Dobzhansky (1973). The American Biology Teacher, 35(3), 125-129.
“1983…A Merman I Should Turn to Be”
The rock & roll connoisseurs amongst you might recognize this as the title of one of the tracks from Electric Ladyland. Electric Ladyland was released by MCA records in 1968 and signaled the coming of age of the Jimi Hendrix Experience; in part because it featured tracks that exceeded the AM radio play limit of 3 minutes and 30 odd seconds. The lyrics of the song are not really relevant to this month’s digression. Rather it is the title that alludes to the notion of intelligent hominid type life forms existing below the ocean. Is it possible that intelligent life can exist in the ocean depths, and if so, are there any limitations to how such life would/could develop in an aquatic environment?
This question takes on even more interest when we consider that recently astronomers found a super-Earthlike planet around a nearby red dwarf. The planet is named GJ 1214b, it is about 19 times as large as Earth by volume but only 6.6 times as massive. The mass differential could be explained if the object were composed primarily of water in liquid form, with a modest amount of rocky material at its core. The researchers have calculated that it must also have an atmosphere. While these conditions might allow for “life” on this world, its proximity to the red dwarf, makes it slightly too hot to be habitable1. However, there is no reason to imagine that there may not be many aquatic planets, at distances from their suns, suitable for the evolution of life (remember that life on Earth mainly exists in our oceans!)
The Drake Equation
The Drake equation was developed to reasonably estimate the distribution of life in the universe, astronomers have developed the formula N = R*fpnef1fiftL. The equation is named after the 20th-century American astronomer Frank Drake, who conducted the first radio search for extraterrestrial intelligence.
In this formula:
|N||the number of civilizations capable of communicating across interstellar space|
|R*||the mean rate of star formation|
|fp||the fraction of stars with interplanetary systems|
|ne||the mean number of planets in each planetary system that are suitable for the origin and evolution of life|
|f1||the fraction of planets on which life can actually develop|
|fi||the fraction of life-bearing planets on which intelligent life has evolved|
|ft||the fraction of planets bearing intelligent life that can give rise to a civilization capable of interstellar communications|
|L||the lifetime of a technical civilization|
Of these figures, only R* is has been reasonably estimated from astrophysical studies. The estimates of the other figures are based largely on experience of the single example thus far known—life on earth. Cirkovic 2004 is somewhat critical of the formulation of the Drake equation. He particularly criticizes the absence of an evolutionary approach in the formulation of this equation. However, using this formula, some astronomers suggest that the number of civilizations in the earth’s galaxy alone may range from a thousand to a million. One can ask how many of these planets might be the home of “aquatic” civilizations?
The Evolution of Life
Life as we know it contains the following core components:
1. Organization [that is organization different from the general characteristics of matter.]
2. Metabolism [utilizes energy to maintain organization, consistent with the laws of thermodynamics.]
3. Self-replicating [no living systems can survive without reproduction, thus all living systems are subject to evolution – descent with modification.]
4. Possess genetic code [on earth that code is DNA, deoxyribonucleic acid, however there is no a-priori reason why DNA would be the code everywhere in space.]
The characteristics of living things on earth [or life as we know it] are:
1. Elemental composition: H, C, O, N, P, S (note position of these elements in the periodic table) and trace elements such as Mg, Fe, etc.
2. Molecules such as: proteins, carbohydrates, fats, and nucleic acids.
3. Cellular organization.
There are probably good reasons for these general characteristics to be conserved in space, at least on planets with similar mass and temperature as found on Earth. For example, Carbon is required to form polymers. The majority of the mass of the universe is still hydrogen and helium. Nuclear processes in star formation create heavier elements such as carbon and nitrogen, although ethanol (CH3-CH2-OH) is formed in interstellar dust clouds. The frequency distribution of heavy elements in the universe follows the inverse-J shaped distribution expected of rare events (illustrated as a broken stick model, formally given by the Poisson distribution: Pr (X = k) = e-m * mk/k!, where K is the number of rare events, and m is the mean number of expected events over a small interval.) How do you know this? The largest amount of chemical matter in the Universe is found in the form of H2 gas. Stars take H2 gas and form He gas through nuclear fusion, however heavier elements such as C, O, Ca, FE are formed and spun out during their formation.
Our sun is composed primarily of H and He gas (99%), the 1% left over contains traces of Fe and other heavier elements. So it seems that the stuff from which planets form is inherited heavier matter from the fusion of earlier stars! This chemical factory occurs in the arms of spiral galaxies, were interstellar dust accumulates, and shields H2 atoms from ultraviolet light, so that molecules like H20, CO, and NH3 form. Over 100 molecules have been found in interstellar dust clouds. Heavy elements such as uranium are formed in the shock waves of supernova explosions.
When a star is formed from cosmic dust (in nebulas) it first formed a disk of gas and dust. When the star condenses, the dust aggregates into rocky planets, such as Earth. Residual gas accumulates to make gas planets, such as Jupiter. Somewhere in between these options we can imagine planets that have smaller amounts of dust and more amounts of water. Earth was formed by the accretion of cosmic dust and meteorites. At about 4.4 billion years ago, the core appeared. The core and mantle drive the geothermal cycle which included volcanism. Gases emerging from the interior produced the early atmosphere. Continental crust formed as different elements segregated to different depths.
Earth changed drastically since its formation. Our planet was constantly bombarded by meteors one hundred million years after it formed (4.35 byp). At this point it studded with volcanic islands and shrouded by an atmosphere composed of CO2, with heavy clouds. About 3 billion years ago, it would have been obscured by an orange haze of methane gas produced by some of the first organisms. The elemental composition of the atmosphere on pre-biotic Earth consisted of water (H2O), hydrogen gas (H2), CH4, and NH3
The Miller-Urey experiments demonstrated that polymerization of carbon compounds was thermodynamically favored under these conditions. They also showed that amino acids could be formed by the molecules found in the Earth’s ancient atmosphere. The basic structure of an amino acid contains a carbon by an amine group, carboxcylic acid, hydrogen atom, and a “R” group which could be any sort of carbon, nitrogen chain.
There are 23 naturally occurring Amino acids, they differ based on the chemical composition of their R group. The R groups can be positively or negatively charged (hydrophilic) or non-charged (hydrophobic.) When amino acids are brought together they will spontaneously bond to form polymers known as polypeptides (these are the building blocks of proteins.) The polypeptides take on a three dimensional structure. These shapes are called their secondary structure and they by necessity catalyze chemical reactions.
Thus, proteins are both autocatalytic (origin of metabolism) and self-replicating (e.g. prions or protein conformation change diseases, Kirchfeld-Jacob disease.) Thus we believe that the evolution of genetic codes probably occurred in reverse order – proteins, RNA, then DNA. The self-replication of genetic codes automatically creates natural selection. In other words, since codes will replicate at different rates, some codes will be favored in the world of self-replication. Chemical mechanisms for replication are not error free (thus mutation) will occur spontaneously.
Mutation is the source of variation in genetic codes (variation is required for any evolutionary theory utilizing natural selection.) Consider the phrase: Dr. Graves sure has some crazy ideas. Do you think that you could type this phrase over 1 million times without making mistakes? Consider how many mistakes you would make if you were constrained to accomplish this, while using the least amount of energy?
Information content of the phrase is given by 3728 power. This is determined by counting the number of letters, periods, and spaces in the phrase and considering that each space could have a total of 28 possible entries. This is 8.12 x 1043 possibilities!
Most of the random changes in the phrase if we replicate it would lead to nonsense phrases. However a small portion of them would produce phrases that are “improvements.”
Consider if we wish to evolve a new phrase:
Dr. Graves sure has some great ideas.
If we had a means to “select” only those reproduced “errors” that would converge on the new phrase, we find that this selection could occur in very short order. (to change crazy to great requires only changing c to g, a to e, z to a, y to t.) The probability of this occurring in any single selected event would be (1/28 * 1/28 * 1/28 * 1/28, or 1.62 x 10-6. However, the probability that this would never happen would be (1.0 – 1.62 x 10-6)x. This number is approximately 0.999999. The variable (x) is the number of generations of replication under selection. This probability approaches 0 as x approaches large numbers. This is simply a way of saying that even rare events must happen.
|Number of replication events||Prob. of Never happening|
The evolution of the simplest living things on earth took at least 1 billion years (represented by organisms we know as Archaea.) This is however an underestimate (not utilizing the time scale at which molecular events occur.) Even if we used the time scale of bacterial replications, 30 minutes per generation, we would arrive at: 1.752 x 1013 generations. Remember that this figure is also an underestimate of molecular scale. Thus there would have been ample time for the primitive earth to evolve by the means of natural selection, self-replicating molecular systems, eventually converging on systems like today’s modern cells.
Characteristics of intelligent life on Earth
To understand how intelligent life might evolve on an aquatic planet, we might ask how this occurred on Earth. Organisms that have intelligence (however defined) share the following features:
Good neural development
Closed circulatory system*
Muscular, large heart*
May or may not be conserved in space*
These characteristics suggest that intelligent life forms should be rare, even in the history of life on this planet. For example, the vast majority of living things on this planet are single-celled. Of those that are not, we see intelligence only occurring within one major taxonomic category: Animalia (the animals.) Intelligence is quite rare within the animals, comprising 35 phyla, only two show organisms with intelligence (Mollusca; particularly class Cephalopoda and Chordata; particularly class Aves (Birds) and Mammalia (Mammals.) Thus, within all the animal species, on our planet, intelligence is rarely observed.
Problem solving intelligence has been long-observed in the Cephalopods. Octopuses presented with a challenge of obtaining live crabs from sealed glass jars learn how to successfully extract the crabs by unscrewing the lids. The more opportunities they are presented to do this, the faster they learn to achieve the task2. Recently, tool use has been observed in Octopuses. The veined octopuses, Amphioctopus marginatus has been filmed picking up coconut halves from the seabed to use as hiding places when they feel threatened3. You can see the video of this at http://www.newscientist.com/article/dn18281-octopuses-use-coconut-shells-as-portable-shelters.html ). Amongst the Chordates, Whales (Order: Cetacea) have shown great intelligence, however these organisms are limited in their ability to manipulate their environment due to the fact that their appendages form fins (adapted for swimming) as opposed to “hands” or tentacles in the case of the octopus.
How far could intelligent life go in an ocean?
One can imagine that somewhere in the universe there are highly intelligent aquatic organisms. But would we ever expect Mermen or technology wielding and space traveling whales as conceived of in Star Trek: Enterprise? Seasons 3 and 4 of Enterprise revolve around an attack on Earth conducted by the Xindi. The Xindi are from a world that includes five intelligent species (including two primate-like, one whale-like, one reptilian, and one insectoid.) Of these, I find the idea of a technology wielding whale species most problematic, even though I concede the idea that intelligence in whales is quite highly developed. The problem with the development of high technology in the ocean is the absence of fire. A key milestone for the development of human civilization was the ability to smelt metals and fashion iron, bronze, copper, and steel. These metals made possible a wide range of tools that I cannot imagine a technological society without. Thus the only scenario under which a highly technically advanced society might exist in the oceans, is if the technology was first developed by land inhabiting organisms, which for some reason retained the ability to live in the ocean. Indeed, this is the premise of the Hendrix ballad. Humans forced to adapt to the life in the ocean, due to disaster which was making the land environment no longer desirable.
“Hurray I awake from yesterday
Alive but the war is here to stay
So my love Catherina and me
Decide to take our last walk thru the noise to the sea
Not to die but to be reborn
Away from the lands so battered and torn
Oh say can you see it’s really such a mess
Every inch of Earth is a fighting nest
Giant pencil and lipstick-tube shaped things
Continue to rain and cause screamin’ pain
And the arctic stains from silver blue to bloody red
As our feet find the sand
And the sea is straight ahead
Straight up ahead
Well it’s too bad that our friends can’t be with us today
Well it’s too bad
The machine that we built
Would never save us that’s what they say
That’s why they ain’t comin’ with us today
And they also said it’s impossible
For a man to live and breathe underwater
Forever was a main complaint
Yeah and they also threw this in my face they said
Anyway you know good and well
It would be beyond the will of God
And the grace of the king
Grace of the king
From: 1983…(A Merman I Should Turn to Be), Jimi Hendrix, Electric Ladyland, MCA Records.
1. David Charbonneau et al., A super-Earth transiting a nearby low-mass star; Nature 462, 891-894 (17 December 2009) | doi:10.1038/nature08679; Received 20 October 2009; Accepted 17 November 2009.
3. Finn, J.K, Tregenza, T, and Norman, M.D, Defensive tool use in a coconut carrying Octopus, Current Biology, vol. 19, R1069, 2009.